Using the open top chamber as a cuvette, we have been measuring the effect of elevated CO2 on ecosystem gas exchange. One of the most surprising results in our comparison of inter-annual variation has been the observation that the relative effect of elevated CO2 gross ecosystem production (GEP), essentially photosynthesis of the marsh, did not increase in periods of drought (i.e during 1992,1995, and 1999, See figure below).

Time course for the effect of elevated atmospheric CO2 on annual Gross Ecosystem Production (GEP). GEP is mediated by annual rainfall. During the critical dry years of 1992, 1995, & 1999, NEE was reduced and the CO2 effect was small. This is consistent with greater acclimation during periods of stress (Table 1).

Here GEP is the integrated gross ecosystem production for the period 1988-1999. Thus the value for each year is the integrated ecosystem net photosynthesis for that growing season. We expected to see a stimulation of photosynthesis in those years of low rainfall, 1992, 1995, and 1999 because we had seen a stimulation of shoot density during drought.

The result was very surprising: years of low rainfall were also the years of the smallest effect of elevated CO2 on photosynthesis. This means that drought does not interact with the effects of elevated CO2 on photosynthesis in the same way it interacts with CO2 effects on shoot density and biomass accumulation.

The relationship between periods of drought
(as indicated by peaks in salinity) and the stimulation in C3 shoot density caused by elevated CO2.

A. Shoot biomass at the peak of the growing season and total annual rainfall for the 17 year period, 1988-2003.